Process for the after-treatment of phosphate coatings

ABSTRACT

A PROCESS FOR THE AFTER-TREATMENT OF PHOSPHATIZED METAL ARTICLES, WHICH PROCESS COMPRISES TREATING THE PHOSPHATIZED SURFACE OF THE METAL ARTICLE WITH A NON-AQUEOUS COMPOSITION CONTAINING A SULFUR NOVOLAC RESIN HAVING A MOLECULAR WEIGHT OF FROM ABOUT 300 TO ABOUT 5,000 AND THEREAFTER HEATING THE TREATED METAL ARTICLE AT A TEMPERATURE OF AT LEAST ABOUT 190*C. IN THE PRESENCE OF AN OXYGEN-CONTAINING ATMOSPHERE. THE ARTICLES SO TREATED ARE RENDERED EXCEPTIONALLY RESISTANT TO CORROSION AND TO UNDERCUTTING WHEN COATED WITH FINAL FINISH COATINGS.

United States Patent 3,684,587 PROCESS FOR THE AFTER-TREATMENT OF PHOSPHATE COATINGS Emil J. Geering, Grand Island, and Malcolm H. Shatz, Williamsville, N.Y., and Edward Leon, Parkersburg, W. Va., assignors to Hooker Chemical Corporation, Niagara Falls, N.Y. No Drawing. Filed Dec. 28, 1970, Ser. No. 102,259 Int. Cl. C23f 7/00' US. Cl. 148--6.15 R 7 Claims ABSTRACT OF THE DISCLOSURE A process for the after-treatment of phosphatized metal articles, which process comprises treating the phosphatized surface of the metal article with a non-aqueous composition containing a sulfur novolac resin having a molecular weight of from about 300 to about 5,000, and thereafter heating the treated metal article at a temperature of at least about 190 C. in the presence of an oxygen-containing atmosphere.

The articles so treated are renderedexceptionally resist ant to corrosion and to undercutting when coated with final finish coatings.

This invention relates to the chemical treatment of metal articles. More particularly, this invention relates to the chemical treatment of metal articles to render the metal articles exceptionally corrosion-resistant, -.better' suited to retain final finish coatings, and resistant: to undercutting when final finish coatings corrosive conditions.

It is well known that bonding coatings are desirable on metal articles to protect such articles from corrosion.

do not provide sufiicient protection against the corrosion of the metal in many cases. In order to obtain effective protection against corrosion, the phosphatizing process has been followed in the past, in most instances, by rinsing with dilute chromate solutions.

However, this chromate after-rinsing or after-treatment of the phosphate coating has given rise to, among otherv things, serious disposal problems relative to the resulting Waste water. Because of the toxicity of the hexavalent and trivalent chromium, these materials mustbe removed almost quantitativelyfrom the waste water prior to disposal. Such recovery processes as must be practiced render the chromate rinse economically less desirable. Further, the chromic acid concentrates used for preparing and replenishing the rinsing bath present additional difiiculties in handling due to their strongly corrosive properties. The use of chromate rinses, in some instances, also gives rise to other practical difiiculties. During some applications,

excessiveamounts may tend to either discolor or to bleed are ruptured under ice through subsequently applied topcoats. In other applica tions, the solutions may also tend to run or to pool in confined areas of the metal article workpiece, which may cause subsequent loss of topcoat adhesion when the completed article is subjected to high humidity conditions. Because of the running and pooling of the chromate rinsing solutions, certain areas of the metal articles. are inherently left deficient in rinse treatment. Such areas exhibit as a result poor corrosionresistance and poor resistance to undercutting when exposed to corrosive atmospheres.

Attempts to eliminate or to modify the sealing chromium rinse by post-treating the phosphatized metal articles with solutions of such coating materials as alkene phosphinic acids, acrylics, epoxies and the like have not resulted in the production of entirely satisfactory corrosion-resistant metal articles, generally, having the anti-corrosive and paint-adhesion properties attributed to chromiumrinsed phosphatized metal substrates.

It is an object of the present invention to provide for protective coatings for corrodible metal articles.

It is a further object of the present invention to provide protective coatings for corrodible metal articles which are receptive to retaining final finish coatings.

It is yet another object of the present invention to provide chemically reactive metal-treating composition which will, when applied to corrodible metal articles, render such articles exceptionally corrosion-resistant.

These and other objects will become readily apparent in view of the following description of the present invention.

' According to the present invention, an anti-corrosive and paint-receptive coating is formed on a metal article by the process comprising:

(a) Coating the metal article with a conventional phosphatizing solution;

(b) Treating the phosphatized surface thereof with a non-aqueous solution of a sulfur novolac resin having a molecular weight of from about 200 to about 5,000; and (c) Thereafter heating the treated metal article at a. temperature of at least about C. in the presence of an oxygen-containing atmosphere.

The use of phenolic resins to prevent corrosion and to provide a measure of protection to metal articles is well known and established in the art. However, such value has been achieved by the use of very high molecular weight phenolic resins forming relatively thick, contiguous films, the molecular weight of such phenolic resins varyingfrom about 20,000 to greater than 100,000.

It has now been found that'very low molecular Weight sulfur novolac resins applied to phosphatized metal articles in amounts of from about 4 to about 400 milligrams of resin per square foot of surface area, and generally from the phosphate and the low molecular weight sulfur novolac containing compositions are heated at temperatures in excess of about 190 Cain the presence of oxygen, the resins tend to exhibit chemical, rather than contiguous, behavior. It is believed that, at temperatures in excess of about 190 C., the low molecular weight sulfur novolac 3 resins complex and react with the phosphate crystals and iron oxides present to form caustic-insoluble complexes, and that it is this caustic-insolubility of the formed complexes which provides for the excellent undercutting and corrosion resistance.

The sulfur novolac resins which have been employed with success in the process of the present invention are those resins which may be generally depicted as having the formula:

( n! I- (])11 (|)I-I wherein n is an integer having a value of from 1 to about 40, and x is an integer having a value of from 1 to 10. Such resins are substantially phenol-terminated chain polymers in which the phenolic nuclei are united by sulfur bridges, for the most part located ortho to the phenolic hydroxyl groups.

Such resins are further characterized as permanently soluble and thermoplastic, and curable to insoluble, infusible products upon the application of heat.

The resins are of adequate hardness, with a softening temperature of from about 75 to 125 C., to permit grinding thereof, exhibit good curing properties including a fast rate of cure, and attain a high degree of strength upon being cured.

The sulfur novolac resins finding utility in the process of the present invention may be produced by reacting elemental sulfur with phenol in the presence of base at elevated temperatures. A suitable process is set forth below.

PREPARATION OF S'UIJFUR NOVOLAC RESIN A mixture of 680 grams (7.2 mole) of phenol, 115 grams (3.6 mole) of elemental sulfur and 0.8 gram of sodium hydroxide was heated in a reaction vessel for 13 hours at a temperature of from 150 to 187 C., during which period 58 grams (1.7 mole) of hydrogen sulfide was given off. The reaction mixture was then stripped under a reduced pressure of 20 mm. mercury to yield a sulfur novolac having a sulfur content of 18 percent and a molecular weight of about 268.

The molecular weight and sulfur content can be varied to produce other suitable sulfur novolac resins for use in the present process by varying the proportions of reactants, reaction time, reaction temperature and the like.

Application of the sulfur novolac resin to the phosphatized metal article to be treated is preferably accomplished by solution application. Suitable solvents which may be used include such organic solvents as aliphatic ke'tones such as acetone, methyl ethyl ketone and the like; aliphatic alcohols such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, tertiary butyl alcohol, and the like; aromatic hydrocarbons such as benzene, toluene, xylene and the like; halogenated hydrocarbons such as carbon tetrachloride, trichloroethylene, trichloroethane, tetrachloroethane, perchloroethylene and the like. Generally, those organic solvents which are inert under the conditions of the present process may be employed. Acceptable, generally, are those solvents having boiling points of less than about 150 C. The preferred solvent for the purposes of the present process is acetone.

The non-aqueous sulfur novolac resin treating solution is generally formulated in such a manner as to provide a solution containing from about 0.5 to about 10 percent by weight of the sulfur novolac resin. Preferably, those solutions containing from about 1 to about percent by weight of sulfur novolac resin are employed. However, the use of more or less concentrated solutions is not hereby precluded.

The non-aqueous resin treating solutions may also contain such as coloring agents, anti-smudging agents and the like, generally in amounts not exceeding about 5 percent by weight.

The sulfur novolac treating solution may be applied to the phosphatized metal article either by spraying or by immersing the article in the solution, or by any other suitable means, to ultimately obtain a resin deposit in an amount of less than about 400 milligrams per square foot on the phosphatized metal surface. After the metal article has been treated, the solvent is removed by any suitable means, such as flashing, and the article is then subjected to heating at a temperature in excess of about 190 0., generally at a temperature of from about 190 to about 270 C., and preferably at a temperature of 205 C., in the presence of an oxygen-containing atmosphere, for a period sufiicient to tinich the treatment, generally for a period of about 15 seconds to about 5 minutes.

The time required to effect the heating step is, of course, dependent upon the makeup of the resin treating composition, the temperature employed and the nature of the article being treated. Generally, at a temperature of about 205 C., a finished treatment is effected in less than 180 seconds.

In another variation of the present process, the phosphatized metal article is immersed in the sulfur novolac resin treating solution, removed and heat-treated, then subjected to a final water rinse.

The treated surface of the metal article is highly adher-, ent, receptive to and retentive to the application of paints, varnishes and lacquers. Thus, metal articles, treated according to the process of the present invention, may receive organic finishes having an end use as automobile finishes, exterior trim, coatings on metal building panels, appliance coatings, metal furniture coatings and the like.

Metal surfaces which may be treated according to the process of the present invention include the ferrous metals, aluminum, galvanized metals, alloys and the' like.

The deposition of the phosphate coating may be effected by any of the conventional phosphatizing procedures. Aqueous phosphatizing solutions are generally prepared by dissolving in water small amounts of phosphoric acid and a metal salt such as a nitrate, phosphate, nitrite, sulfate, chloride or bromide of sodium, manganese, zinc, cadmium, iron, lead, nickel, copper or aluminum. Preferably the metal salt employed is one which, in aqueous solution, will yield zinc or iron ions. An oxidizing agent such as sodium chlorate, sodium perborate, sodium nitrite, potassium perchlorate, potassium permanganate or hydrogen peroxide can be included in the phosphatizing solution to depolarize the metal surface being treated and thereby increase the rate at which the phosphate coating is formed on the surface of the metal article.

Other auxiliary agents such as anti-smudging agents, coloring agents, metal cleansing agents and the like may also be incorporated in the phosphatizing solution. A common type of commercial phosphatizing bath which contains zinc ions, phosphate ions, and a depolarizer is made by dissolving small amounts of zinc phosphate, sodium nitrate and phosphoric acid in water.

The phosphatizing solutions suitable for use in the process of the present invention can also be formulated conveniently by first dissolving zinc nitrate, and the like in suflicient water to yield the desired concentration of zinc ions and thereafter adjusting the acidity with phosphoric acid. Another type of commercial phosphatizing bath is one containing nitrate ions, zinc ions, phosphate ions and either nitrite or chlorate ions as a depolarizer.

However, the present process is not dependent upon the use of a particular phosphatizing solution, and any of the several commercial aqueous phosphatizing solutions commonly used in the art may be employed to phosphatize the surfaces of metal articles to be treated according to the present process. Preferred commercial phosphatizing solutions are those which, on being deposited onmetal substrates, will yield zinc or ferrous metal phosphates.

in the accepted practice of phosphatizing a metal article, the surface thereof is usually cleansed by physical and/ or chemical means such as immersion in or spraying with an aqueous caustic cleanser, mechanical abrading or polishing, vapor degreasing and the like. For the purpose of the present process, it is preferred to cleanse the metal article to be phosphatized and subsequently treated with the novolac treating solutions by immersing the article for a period of about 60 seconds in a commercial aqueous custic metal cleanser at a temperature of about 60 C. [Following the cleansing of the metal article, the article is generally rinsed with hot water to prevent contamination of the phosphatizing solution.

The application of the phosphatizing solution to the metal article prior to the application of the sulfur novolac resin treating solution may be accomplished by any suitable means known to the art. Because the phosphatizing solutions are generally applied to temperatures on the order of from about 50 to about 85 C., spray-phosphatizing techniques are suitable for phosphatizing metal articles. Immersion techniques are also well-adapted to providing phosphate coatings on metal articles to be subsequently resin treated. The manner of application, however, is not important in that quiescent baths can be used, or continuous baths may be advantageously employed.

The aqueous phosphatizing solution is generally maintained at temperatures on the orderof from about 50 to about 85 C. Preferably, temperatures on the order of from about 60 to about 70 C. are employed.

Satisfactory phosphate coatings, when applied in accordance with the procedures previously detailed, result in about 60 seconds. Longer or shorter times may be realized by varying the concentration of the phosphatizing solution and the temperature.

Upon completion of the phosphatizing step, the phosphate coated metal article is rinsed with cold water.

It has been found that the wet phosphate coated articles, following the rinsing step, may be immediately treated with the non-aqueous sulfur novolac resin treating solution, and that it is not necessary to remove the water present, whether present as a result of the application of the aqueous phosphatizing solution or as a result of the post-phosphatizing cold water rinse. While the phosphatized metal articles may be subjected to the application thereto of the novolac resin treating solution, no adverse effect on the finished coating, or on the subsequently applied topcoat, results when the procedure of the present process is followed, and the post-rinse drying is omitted. For example, the wet phosphatized article is immersed in a trichloroethylene-sulfur novolac resin treating solution at about 80 C. The water comes off the metal in the trichloroethylene vapor as an azeotrope. As the article is removed, the trichloroethylene flashes off. The mixture of trichloroethylene and water vapors may then be condensed and collected by any suitable means, separated and the trichloroethylene recycled to the resin treating step, and the treated article subjected to the heating step.

Generally, however, it is preferred to subject the phosphatized metal article to a drying step of from about 100 to about 160 C. for a period of from about 30 to about 120 seconds following the rinse and prior to the application of the sulfur novolac resin treating composition thereto.

The temperature of the sulfur novolac resin treating composition is preferably maintained at from about 80 to about 90 C. during application to the phosphatized metal article.

The following examples serve to illustrate the present invention but are not to be construed as limiting it thereto.

Following the resin treatment of the metal articles in the following examples, each article was topcoated and then subjected to percent salt spray and physical tests.

The salt spray test is the American Society for Testing and Materials (ASTM) test B117-61 withpainted panels scribed as given in ASTM test D-16546l. This employs a 5 percent sodium chloride fog or spray. The ratings given depend on the creepage from the scratch, given in 1 of an inch. Ratings given as spt (S) indicate no creepage except in a small area. In the salt spray test, unless otherwise indicated, the exposure time was 120 hours. In the physical test, adhesion was determined by knife blade and the results are reported on the scale of 0 to 10, where 10 is excellent, 8 is good, 6 is fair, 4 is poor, 2 is very poor, 0 is complete loss of adhesion.

Example 1 A polished steel panel is cleansed for 60 seconds in a commercial hot caustic metal cleanser, rinsed with hot water and then dipped for a period of 3 minutes in a commercial zinc phosphate phosphatizing solution. The coated steel panel is then cold water rinsed, flash dried and immersed for 60 seconds in a 5 percent by weight solution of base catalyzed 80-95% ortho-sulfur novolac (MW 1000) in acetone. During the immersion, the temperature of the resin treating solution is maintained at about 25 C. The panel is removed from the resin treating'solution, flashed dry, and heated at a temperature of about 205 C. for a period of seconds in an air circulating oven. The treated panel is found to have deposited thereon less than 400 milligrams of resin per square foot of panel surface area. The treated panel is coated with commercial melamine alkyd enamel. This panel has a knife blade adhesion rating of 10, an undercutting of less than li of an inch and a paint retention of 99 percent.

Example 2 A polished steel panel is cleansed for 60 seconds in a commercial hot caustic metal cleanser, rinsed with hot water, and dipped for 3 minutes in a commercial zinc phosphate phosphatizing solution. The coated steel panel is then cold water rinsed, flash dried and immersed for 60 seconds in a 5 percent by weight solution of base catalyzed 80-95% orthosulfur novolac (MW 482) in acetone, the resin having a sulfur content of 23.72 percent by weight. During the immersion, the temperature of the resin treating solution is maintained at about 25 C. The panel is removed from the resin treating solution, flashed dry, and heated at a temperature of about 205 C. for a period of 150 seconds in an air circulating oven. The treated panel is found to have deposited thereon less than 400 milligrams of resin per square foot of panel surface area. The treated panel is coated with commercial melamine alkyd enamel. The panel exhibits excellent properties, having a knife blade adhesion rating of 10, an undercutting of less than of an inch and a paint retention of 99 percent.

Example 3 The procedure of Example 2 is observed with the exception that the sulfur novolac resin has a molecular weight of 689 and a sulfur content of 24.9 percent by weight. Similar excellent results obtain as in Example 2.

Example 4 The procedure of Example 2 is observed with the exception that the sulfur novolac resin has a molecular weight of 928 and a sulfur content of 26.2 percent by weight. Similar excellent results obtain as in Example 2.

Example 5 The procedure of Example 2 is observed with the exception that the sulfur novolac has a molecular weight of 1214 and a sulfur content of 26.13 percent by weight. Similar excellent results obtain as in Example 2.

Example 6 The procedure of Example 2 is observed with the exception that the resin treating solution contains 1 percent by weight of sulfur novolac resin (MW 700, S 24.26 wgt. percent). Similar excellent results obtain as in Example 2.

Example 7 The procedure of Example 6 is observed with the exception that the sulfur novolac resin has a molecular weight of 580 and a sulfur content of about 27 percent by'weight. Similar excellent results obtain as in Example 2.

Example .8

I The procedure of Example 6 is observed with the exception that the sulfur novolac resin has a molecular weight of about 671 and a sulfur content of about 3.3 percent by weight. Similar excellent results obtain as in Example 2.

Example 9 The-procedure of Example 6 is observed with the exception that the sulfur novolac was the sulfur containing resin resulting from the cooking of sulfur with a random novolac (MW 1000) to a sulfur content of 3.7 percent by weight. Similar excellent results obtain as in Example 2.

Example 10 The procedure of Example 6'is observed with the exception that the sulfur novolac has a molecular Weight of about 248 and a sulfur content of 15.7 percent by weight. Similar excellent results obtain as in Example 2.

Example 11 a, molecular weight of from about 300 to about 5,000,

and a solvent therefor, and thereafter heating the treated metal article at a temperature of at least about 190 C. in the presence of an oxygen-containing atmosphere.

2. .A process as defined by claim 1 wherein acetone is employed as the solvent in the sulfur novolac solution.

3. A process as defined by claim 1 wherein the sulfur novolac resin has a molecular weight of about 1000.

4. A process as defined by claim 1 wherein the treated metal article is heated at a temperature of from about 190 to about 270 C. for a period of from about 15 to about 180 seconds. r

5. A process as defined by claim 1 wherein the resin deposited on the metal article in an amount of from about 4 to about 400 milligrams per square foot of surface area.

6. A process as defined by claim 1 wherein the solvent of the resin treating solution is trichloroethylene.

, 7. A process as defined by claim 1 wherein the sulfur novolac resin is present in the resin treating solution in an amount of from about 1 to about 10 percent by weight.

References Cited, v UNITED STATES PATENTS 2,703,768 3/195 5 Hall 148-6.15 R 3,320,212 5/1967 Shen et al. 26014 X 3,185,666 5/1965 Harding et al. 260l7.2 X

RALPH S. KENDALL, Primary Examiner C. WESTON, Assistant Examiner us. 01. X.-R. 1i7 132 BF, L, 161 L 

